It is now common practice to perform seismic exploration of the earth by imparting an acoustic wave to the earth at a first location and detecting the return of the wave after reflection from subterranean rock formations at plural additional locations. The signals output by the detectors, usually referred to as "geophones" when used in earth-based exploration and as "hydrophones" in ocean-going operations, can then be processed in known fashion to generate representations of the interfaces between rock layers of varying density which then can be used by geophysicists in the search for oil and gas.
To date, all geophones of which the present inventor is aware are generally similar in that they contain a single magnetized mass oscillating in a coil in which there is generated a voltage in response to the oscillation. The detector is coupled to the earth, so that it oscillates in response to the returned wave of seismic energy. The magnetic mass is typically spring loaded and its oscillatory motion is damped by a dashpot or similar device. Any such system has a frequency response which is nonlinear, and therefore the voltage generated in the coil, which is recorded as the seismic signal, will bear a similarly nonlinear relationship to the actual received seismic signal as well. It is believed that it would be desirable to provide a geophone having a more balanced frequency response, as this might enable subsequent data processing to yield a more realistic depiction of the earth's structure. It would also be desirable to provide a geophone in which the different frequency components of the output signal could be separately generated at the geophone, rather than in subsequent processing steps, for increased accuracy in discrimination between various types of seismic events exhibiting frequency dependency.